Electronic devices have been elaborated which are fabricated by forming functional films of oxides or nitrides on silicon substrates or semiconductor crystal substrates, followed by integration. Attempts have been made, by combining semiconductor substrates with dielectric thin films, to fabricate dielectric isolated LSI devices having a higher degree of integration than relying on the LSI or silicon-on-insulator (SOI) technology. Using thin films of ferroelectrics belonging to dielectrics, non-volatile memories, infrared sensors, optical modulators, optical switches, and OEIC can be constructed. From the combination of semiconductor substrates with superconductor thin films SQUID, Josephson devices, superconducting transistors, electromagnetic wave sensors and superconductor wired LSI can be constructed. From the combination of semiconductor substrates with piezoelectric thin films, SAW devices, convolvers, collimators, memory devices, image scanners, thin film bulk resonators and filters can be constructed.
To ensure optimum device characteristics and reproducibility thereof for semiconductor devices using such functional films, single crystals are preferably used as the semiconductor substrates. With polycrystalline materials, it is difficult to provide good device characteristics on account of the disturbances of physical quantities by grain boundaries. The same applies to functional films. It is desired that the functional films be epitaxial films which are as close to single crystals as possible.
Most piezoelectric materials which are of worth in applications have a crystal structure of the wurtzite type as typified by ZnO and AlN. Since the epitaxial growth of wurtzite type compounds largely depends on the crystallographic orientation of substrate materials, it is difficult to perform direct epitaxial growth on a silicon single crystal substrate which is of the cubic crystal system.
An attempt to perform direct epitaxial growth of a ZnO thin film on a silicon substrate results in the formation of an SiO.sub.2 layer on the surface of the silicon substrate. During growth of ZnO crystals, the presence of an SiO.sub.2 layer prevents the configurational information of silicon crystals from being conveyed. As a result, the ZnO thin film is grown as a polycrystalline film of c-axis orientation. For this reason, the epitaxial growth of a ZnO thin film on a silicon substrate has never been reported.
Several reports are found with respect to the direct epitaxial growth of an AlN thin film on a silicon substrate. For example, Jpn. J. Apl. Phys., vol. 20, L173 (1981) reports that using MOCVD (chemical vapor deposition using organic metal), AlN is epitaxially grown on a silicon substrate at a temperature of 1,260.degree. C. There are also found research reports using AlN as the buffer layer for GaN base light-emitting devices. For example, Technical Research Report of the Electronic Communications Society, CPM92 1-13, p. 45 (1992) describes the possibility of epitaxial growth above 1,100.degree. C. using MOVPE (organic metal vapor phase growth).
In either case, the growth temperature of AlN thin films is as high as 1,100.degree. C. or above. When an AlN thin film is formed above 1,000.degree. C., aluminum which constitutes AlN tends to form aluminum silicide by reacting with the silicon substrate. Therefore, when the forming temperature is above 1,000.degree. C., a careful attention must be paid so as to suppress the formation of aluminum silicide, adversely affecting the abilities of mass scale manufacture and reproduction.
Also, thin films of Group III-V nitride semiconductors such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), aluminum gallium nitride (AlGaN) and mixed crystals thereof are now utilized in nitride semiconductor devices such as field effect transistors, light-emitting diodes (LED), and laser diodes. As described in Nikkei Electronics, No. 674, p. 79 (1996), LEDs having a multilayer structure of nitride semiconductor layers and emitting light of a short wavelength such as blue or green light are of interest.
In semiconductor devices using Group III-V nitride semiconductor layers, for example, GaN thin films, sapphire is generally used as the substrate on which GaN thin films are formed. However, since sapphire has a lattice constant and a coefficient of thermal expansion largely differing from those of GaN, there arises the problem that crystals of quality are not obtained owing to the introduction of dislocations from the interface between the substrate and GaN to the GaN side or the deformation of GaN crystals by stresses. Sapphire substrates are difficult to break so as to expose the cleavage plane, and it is then difficult to form the end face in the manufacture of laser diodes. Sapphire substrates have further problems that they are expensive compared with silicon and other semiconductor substrates and poor in surface flatness. A still further problem of sapphire substrates is the lack of electric conductivity.
On the other hand, because of great differences in lattice constant, coefficient of thermal expansion and lattice structure between silicon and GaN, it is difficult to form GaN thin film of quality on silicon single crystal substrates.
As an attempt for improving the crystallinity of thin films of nitride semiconductors such as GaN, JP-A 45960/1997, for example, discloses the formation of a InGaAlN layer on a sapphire or silicon substrate with a ZnO buffer layer disposed therebetween. In this patent, the ZnO buffer layer is directly formed on the silicon substrate as by sputtering. However, according to our follow-up test, it is substantially impossible to form a ZnO buffer layer on a silicon substrate as a single crystal film (or epitaxial film as used in the present disclosure). The film cannot be a film having good crystallinity and surface flatness. It is then impossible to form on such a ZnO buffer layer, a nitride semiconductor layer having good crystallinity.
Further, JP-A 264894/1996 discloses a semiconductor device comprising a silicon or silicon carbide substrate, at least one of a Ca.sub.x Mg.sub.1-x F.sub.2 layer (0.ltoreq.x.ltoreq.1) and a Mg.sub.t Ca.sub.3-t N.sub.2 layer (0.ltoreq.t.ltoreq.3) formed thereon, and a Ga.sub.y In.sub.z Al.sub.1-y-z N layer (0.ltoreq.y, z.ltoreq.1) formed thereon. The alleged advantages are that silicon or silicon carbide substrates having good surface flatness can be used and GaInAlN layers of quality can be formed. However, our follow-up test revealed that the Ca.sub.x Mg.sub.1-x F.sub.2 layer or Mg.sub.t Ca.sub.3-t N.sub.2 layer formed on the silicon substrate was insufficient in crystallinity and surface flatness. It is then impossible to form on such a layer, a nitride semiconductor layer having good crystallinity.
It is also known from J. Cryst. Growth, 128, 391 (1993) and J. Cryst. Growth, 115, 634 (1991) to use an AlN thin film or SiC thin film as the buffer layer when a GaN thin film is formed on a silicon substrate. However, our follow-up test revealed that the AlN thin film or SiC thin film formed directly on the silicon substrate was insufficient in crystallinity and surface flatness. It is then impossible to form on such a thin film, a nitride semiconductor layer having good crystallinity.